Method and apparatus for setup of a circuit switched call during circuit switched fallback

ABSTRACT

A method, at a user equipment (‘UE’), the method: sending a request to a first network for Circuit Switched Fallback (‘CSFB’); receiving, responsive to the request, redirection information; acquiring a second network based on the redirection information; initiating establishment of Circuit Switched (‘CS’) voice radio bearers with the second network; waiting for a predetermined event to occur; after the predetermined event has occurred, establishing a Packet Switched (‘PS’) connection with the second network. Further, a method, at a network element, the method receiving a request for Circuit Switched Fallback (‘CSFB’) from a user equipment (‘UE’); sending, to the UE, redirection information; receiving a Routing Area Update (‘RAU’) message from the UE; waiting for a predetermined event to occur; upon the predetermined event occurring, responding to the RAU message.

RELATED APPLICATIONS

The present application is a continuation of U.S. Pat. No. 9,326,194,issued Apr. 26, 2016, and filed Sep. 11, 2013, the entire contents ofwhich are incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to the setup of a circuit-switched callon a mobile communication network and in particular relates to the setupof a circuit switched call during circuit switched fallback (CSFB).

BACKGROUND

The Third Generation Partnership Project (3GPP) defines a long-termevolution (LTE) architecture, which provides high data rate, lowlatency, packet optimization and improved system capacity and coverage.In an LTE system, an evolved universal terrestrial radio access network(E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) andcommunicates with a plurality of mobile stations, also referred to asuser equipments (UEs).

An LTE network supports only packet-switched (PS) services. Howeverfallback is specified for circuit switched (CS) services as well. CSFBdefines a mechanism for using a legacy CS network (e.g. Universal MobileTelecommunications System Terrestrial Radio Access Network (UTRAN) or aGSM EDGE Radio Access Network (GERAN)) to provide voice and traditionalCS-domain services (e.g. voice call, a CS short message service (SMS),Unstructured Supplementary Service Data (USSD), among others).

With CSFB, when there in an incoming CS call for a UE operating in LTEmode, the LTE network pages the device. The device responds with aspecial service request message to the network, and the network signalsthe device to move (fall back) to GERAN/UTRAN cell by providingredirection information for the UE to acquire a UTRAN/GERAN cell.Similarly for outgoing CS calls, the same special service request isused to move the device to GERAN/UTRAN to place the outgoing call. TheUE then attempts to acquire a GERAN or UTRAN cell based on the receivedredirection information and then attempts to establish a connection.

The UE then attempts to establish CS service and may attempt to movepacket switched services from E-UTRAN to GERAN or UTRAN if applicable.In particular, to maintain data connectivity, an always-on UE isrequired to perform a Routing Area Update (RAU) procedure to transferthe PS data context from E-UTRAN to UTRAN/GERAN. In addition, there mayhave been a data session in progress when the CSFB call was initiated,which would trigger the UE to perform an RAU procedure and then initiatea packet data call in parallel with the establishment of the CS call.

The overall call setup time for a CSFB call is significantly longercompared to a CS voice call that is initiated while the UE is already inGERAN/UTRAN. Further, additional delays are caused if PS domainsignaling activities take place while the CS domain is being setup.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be better understood with reference to thedrawings, in which:

FIG. 1 is a diagram of an example network architecture.

FIG. 2 is a flow chart of a process according to at least one embodimentof the present disclosure.

FIG. 3 is a flow chart of a process according to a further embodiment ofthe present disclosure.

FIG. 4 is a flow chart of a process according to a further embodiment ofthe present disclosure.

FIG. 5 is a flow chart of a process at a network side according afurther embodiment of the present disclosure.

FIG. 6 is a bar chart of the observed call setup times of legacy CScalls, CSFB prior art calls and CSFB calls in accordance with at leastone embodiment of the present disclosure.

FIG. 7 is a block diagram of an example user equipment.

FIG. 8 is a block diagram of an example simplified network element.

DETAILED DESCRIPTION

The present disclosure provides a method, at a user equipment (‘UE’),comprising: sending a request to a first network for Circuit SwitchedFallback (‘CSFB’); receiving, responsive to the request, redirectioninformation; acquiring a second network based on the redirectioninformation; initiating establishment of Circuit Switched (‘CS’) voiceradio bearers with the second network; waiting for a predetermined eventto occur; after the predetermined event has occurred, establishing aPacket Switched (‘PS’) connection with the second network.

The present disclosure further provides a user equipment comprising: aprocessor; a communications subsystem; wherein the user equipment isconfigured: send a request to a first network for Circuit SwitchedFallback (‘CSFB’); receive, responsive to the request, redirectioninformation; acquire a second network based on the redirectioninformation; initiate establishment of Circuit Switched (‘CS’) voiceradio bearers with the second network; wait for a predetermined event tooccur; after the predetermined event has occurred, establishing a PacketSwitched (‘PS’) connection with the second network.

The present disclosure further provides a method, at a network element,comprising: receiving a request for Circuit Switched Fallback (‘CSFB’)from a user equipment (‘UE’); sending, to the UE, redirectioninformation; receiving a Routing Area Update (‘RAU’) message from theUE; waiting for a predetermined event to occur; upon the predeterminedevent occurring, responding to the RAU message.

The present disclosure further provides a network element, comprising: aprocessor; a communications subsystem; wherein the network element isconfigured to: receive a request for Circuit Switched Fallback (‘CSFB’)from a user equipment (‘UE’); send, to the UE, redirection information;receive a Routing Area Update (‘RAU’) message from the UE; wait for apredetermined event to occur; upon the predetermined event occurring,respond to the RAU message.

The overall call setup time for a CSFB call is significantly longercompared to a CS voice call that is initiated while the UE is alreadycamping on GERAN/UTRAN. One major contributor to the increase in callsetup time is the acquisition of GERAN/UTRAN, and subsequentsynchronization with GERAN/UTRAN which is needed before UE can exchangeany signaling message with GERAN/UTRAN.

Further, additional delays may be caused by PS domain signalingactivities that happen during the CS domain setup. In addition to thesignaling messages that are exchanged to set up CS call, if the UEinitiates an RAU procedure, the network in response initiates a PSdomain security procedure which prevents or delays the signaling radiobearers to be used for signaling of CS domain activities until the PSdomain security procedure is completed.

In addition, the network may also initiate the General Packet RadioService (GPRS) Mobility Management (GMM) authentication and cipheringprocess for PS domain which delays the CSFB call setup further.

As a result of all these procedures, the establishment of voice radiobearers for the CS domain is significantly delayed compared to a legacyCS voice call in GERAN/UTRAN. As used herein, the CS voice radio bearersare the radio resources that are setup by the network to transport CSvoice over wireless network and can be for either second or thirdgeneration networks. The end-to-end call setup delay is particularlylong and noticeable by the user if a call is made from one LTE device toanother LTE device when both devices are on an LTE network and bothundergo the CSFB procedures including PS domain transfer when CS voicecall is being setup. This delay in call setup adversely impacts userexperience; in addition, such increase in call setup time is highlyundesirable if the voice call is an emergency call or an emergencycallback call from the Public Safety Answering Point (PSAP).

Circuit switched fallback is provided for in the 3GPP LTE TechnicalSpecification (TS) 23.272, “Third Generation Partnership Project;Technical Specification Group Services and System Aspects; CircuitSwitched (CS) Fallback in Evolved Packet System (EPS); Stage 2”; v.11.5.0, June 2013, the contents of which are incorporated herein byreference. CSFB can be used to fall back to either second generationGERAN networks or third generation UTRAN networks. In particular thesystem architecture is provided in FIGS. 4.2-1 of the 3GPP TS 23.272specification and is reproduced herein with regard to FIG. 1.

Reference is now made to FIG. 1, which illustrates one embodiment of amobile communication system that includes a GERAN/UTRAN network, as wellas an LTE network.

In particular, a UE 110 is a multi-mode UE capable of communicating withboth an LTE network as well as a GERAN/UTRAN network. As seen in FIG. 1,UE 110 communicates with either or both of a UTRAN 120 or a GERAN 122for circuit switched services. UE 110 further communicates with anE-UTRAN 130 for the LTE network.

Both UTRAN 120 and GERAN 122 communicate with a serving general packetradio service (GPRS) support node (SGSN) 140 for packet switchedservices and with MSC server 160 for circuit switched services.

The E-UTRAN communicates with mobility management entity (MME) 150.

In the CSFB system, mobile switching center (MSC) server 160 furthercommunicates with MME 150.

During normal operation, UE 110 will camp on the LTE system. If a mobileterminated call is received then MSC server 160 will notify MME 150,which will then send a page to UE 110 through E-UTRAN 130 to transitionto UTRAN 120 or GERAN 122. The UE 110 will then transition for theduration of the circuit switched call. At the end of the circuit switchcall the UE then transitions back to the LTE network.

Packet switched services may either be suspended for the duration of thecircuit switched call or may be transferred to SGSN 140, where the UE110 will continue to receive packet data, although at a lower speed thanfrom the LTE network.

For a mobile originated call, the UE 110 may transition to UTRAN 120 orGERAN 122 for the duration of the call.

Typical non-access stratum/radio resource control (NAS/RRC) signalingfor a mobile-terminated CS voice call setup between a UE camped on aUTRAN cell and a UMTS network is shown with regards to Table 1 below.The messages shown in italics are optional messages that UMTS networkmay initiative for authentication purposes.

TABLE 1 CS Voice Call for UE camped on UTRAN Cell UE ← Network Paging UE→ Network rrcConnectionRequest (terminatingConversationalCall) UE ←Network rrcConnectionSetup UE → Network rrcConnectionSetupComplete UE →Network initialDirectTransfer (NAS RR PAGING_RESPONSE) UE ← NetworkdownlinkDirectTransfer (NAS MM AUTHENTICATION_REQUEST) UE → NetworkuplinkDirectTransfer (AUTHENTICATION_RESPONSE) UE ← NetworksecurityModeCommand (CS domain) UE → Network securityModeComplete UE ←Network downlinkDirectTransfer (NAS MM IDENTITY_REQUEST) UE → NetworkuplinkDirectTransfer (NAS MM IDENTITY_RESPONSE) UE ← NetworkdownlinkDirectTransfer (NAS CC SETUP) UE → Network uplinkDirectTransfer(NAS CC CALL_CONFIRMED) UE ← Network radioBearerSetup (CS domain RAB) UE→ Network radioBearerSetupComplete UE → Network uplinkDirectTransfer(NAS CC ALERT)

Conversely, Table 2 shows the NAS/RRC signaling during amobile-terminated CS voice call setup, between a UE initially connectedto an LTE network and the networks comprising the LTE network thatreceives CS voice call request and the UMTS network that handles the CSvoice call. The bold messages are for the PS domain. In this example,the UE fallbacks to UTRAN for CS voice call. Fallback to GERAN comprisessimilar steps.

TABLE 2 LTE to CSFB Voice Call LTE to UMTS CSFB Voice Call UE ← NetworkPaging UE → Network EPS MM Extended service request (MT CSFB) UE →Network rrcConnection Request (mt-Access) UE ← NetworkrrcConnectionSetup UE → Network rrcConnectionSetupComplete UE ← NetworksecurityModeCommand UE → Network securityModeCommandComplete UE ←Network ueCapabilityEnquiry UE → Network ueCapabilityInformation UE ←Network rrcConnectionReconfiguration UE → NetworkrrcConnectionReconfigurationComplete UE ← Network rrcConnectionRelease(redirectedCarrierInfo utra-FDD) UE acquires a UTRAN cell UE → NetworkrrcConnectionRequest (terminatingConversationalCall) UE ← NetworkrrcConnectionSetup UE → Network rrcConnectionSetupComplete UE → NetworkinitialDirectTransfer (NAS RR PAGING_RESPONSE) UE → NetworkinitialDirectTransfer (GMM_ROUTING_AREA_UPDATE_REQUEST) UE ← NetworkdownlinkDirectTransfer (NAS MM AUTHENTICATION_REQUEST)

←

UE → Network uplinkDirectTransfer (AUTHENTICATION_RESPONSE)

→

UE ← Network securityModeCommand (CS domain) UE → NetworksecurityModeComplete UE ← Network securityModeCommand (PS domain) UE →Network securityModeComplete UE ← Network downlinkDirectTransfer(GMM_ROUTING_AREA_UPDATE_ACCEPT) UE → Network uplinkDirectTransfer(ROUTING_AREA_UPDATE_COMPLETE) UE ← Network downlinkDirectTransfer (NASMM IDENTITY_REQUEST) UE → Network uplinkDirectTransfer (NAS MMIDENTITY_RESPONSE) UE ← Network downlinkDirectTransfer (NAS GMMGMM_INFORMATION) UE ← Network downlinkDirectTransfer (NAS CC SETUP) UE →Network uplinkDirectTransfer (NAS CC CALL_CONFIRMED) UE ← NetworksignallingConnectionRelease (PS domain) UE ← Network radioBearerSetup(CS domain RAB) UE → Network radioBearerSetupComplete UE → NetworkuplinkDirectTransfer (NAS CC ALERT)

As seen in Table 2 above, the UE is connected to the LTE network priorto acquiring the UTRAN cell, after which the UE is connected to the UMTSnetwork.

Since the UE must first acquire the UTRAN cell, CS call setup with CSFBhas more latency. For example, one set of real world test results, wherethe tests were performed on live-air commercial networks to compare thecall setup latency of a CSFB call compared to a CS voice call, are shownwith regards to Table 3 below. The tests were performed simultaneouslyso that any loading or other artifacts of the network affected each callin a similar manner. The results in Table 3 are merely examples, butshow the call setup time for CSFB calls is significantly longer than forregular CS voice calls.

TABLE 3 Comparison between Call Setup Latencies Median Call Setup Time(ms) Breakdown of Total LTE Ext UMTS RRC Service Connection Request toRequest to UMTS RRC UMTS Radio Source to Connection Bearer SetupDestination Request for CS domain Type Network Type of Call Total(LTE→UMTS) messages Land to Network 1 Legacy CS 1268.5 N/A 1268.5 MobileCall voice call in UTRAN CSFB LTE to 2528 1002  1526 UTRAN NetworkLegacy CS 1545 N/A 1545 2, voice call in location 1 UTRAN CSFB LTE to3406.5   988.5 2418 UTRAN Network Legacy CS 1691.5 N/A 1691.5 2, voicecall in location 2 UTRAN CSFB LTE to 3541 816 2725 UTRAN Mobile toNetwork Legacy CS 3749 N/A 3749 Mobile Call 2, voice call in location 2UTRAN CSFB LTE to 6770 920 5850 UTRAN

Accordingly, the embodiments described below provide solutions to reducethe CSFB call setup delay. Specifically, embodiments of the presentdisclosure provide for the reduction in the CSFB delay by delaying PSdomain signaling activities until after the voice radio bearer for theCS domain is established.

According to at least one embodiment, the PS domain signaling activitiesare suspended until the CS domain voice radio bearer for the voice callis setup. According to at least one further embodiment, the PS domainsignaling activities are delayed for a certain duration.

According to at least one embodiment, these solutions may be triggeredon the mobile device, regardless of whether there is an ongoing activePS data session in LTE at the time of the CSFB call. In an at leastanother embodiment, these solutions may be triggered only when there isno active PS data session in LTE or pending PS data session or requestin UMTS for a PS data session. At the time of CSFB call in LTE, the NonAccess Stratum (NAS) layer on the UE can implicitly determine that theUE has no active PS data session if the Evolved Packet System MobilityManagement (EMM) entity is in idle mode (EMM-IDLE mode). The NAS layeron the UE may also check the status of a follow-on bit in an RAU requestto check if there is any pending data or not.

Notably, the above solutions may delay the transfer of the PS datacontext from LTE to UTRAN by a small margin but reduce the latency forCS voice call setup which is more important from users' perspective.

Reference is now made to FIG. 2, which provides a flow chart of a methodaccording to at least one embodiment of the present disclosure.

The method of FIG. 2 starts at block 200, and proceeds to block 210 inwhich a CSFB call request is sent from the UE to the LTE network. Forexample, the request may be based on a page from the network for amobile terminated call, as seen in Table 2 above, or may be for a mobileoriginated call.

The process next proceeds to block 220, in which the UE receivesredirection information from the network in response to the CSFB callrequest. For example, from Table 2 above, the redirection informationmay be in the form of a rrcConnection Release (redirectedCarrierinfoutra-FDD) message.

Based on the received redirection information, the UE attempts toacquire and then connect to a UTRAN cell to establish circuit switchedservice, as shown at block 230.

The UE then waits until the circuit switched voice radio bearers areestablished, and checks that the circuit switched voice radio bearersare established at block 240. Once the circuit switched voice radiobearers are established, the process proceeds block 250 in which UEinitiates the procedure to transfer the PS domain to UTRAN by sendingthe GMM_ROUTING_AREA_UPDATE_REQUEST to the UTRAN.

The process then proceeds to block 260 and ends.

Table 4 below shows example signaling for a mobile-terminated voice callbetween a UE and a network when the above solution is implemented.

TABLE 4 LET to UMTS CSFB Voice Call (modified) LTE to UMTS CSFB VoiceCall UE ← Network Paging UE → Network EPS MM Extended service request(MT CSFB) UE → Network rrcConnectionRequest (mt-Access) UE ← NetworkrrcConnectionSetup UE → Network rrcConnectionSetupComplete UE ← NetworksecurityModeCommand UE → Network securityModeCommandComplete UE ←Network ueCapabilityEnquiry UE → Network ueCapabilityInformation UE ←Network rrcConnectionReconfiguration UE → NetworkrrcConnectionReconfigurationComplete UE ← Network rrcConnectionRelease(redirectedCarrierInfo utra-FDD) UE acquires a UTRAN cell UE → NetworkrrcConnectionRequest (terminatingConversationalCall) UE ← NetworkrrcConnectionSetup UE → Network rrcConnectionSetupComplete UE → NetworkinitialDirectTransfer (NAS RR PAGING_RESPONSE) UE ← NetworkdownlinkDirectTransfer (NAS MM AUTHENTICATION_REQUEST) UE → NetworkuplinkDirectTransfer (AUTHENTICATION_RESPONSE) UE ← NetworksecurityModeCommand (CS domain) UE → Network securityModeComplete UE ←Network downlinkDirectTransfer (NAS MM IDENTITY_REQUEST) UE → NetworkuplinkDirectTransfer (NAS MM IDENTITY_RESPONSE) UE ← NetworkdownlinkDirectTransfer (NAS CC SETUP) UE → Network uplinkDirectTransfer(NAS CC CALL_CONFIRMED) UE ← Network radioBearerSetup (CS domain RAB) UE→ Network radioBearerSetupComplete UE → Network initialDirectTransfer(GMM_ROUTING_AREA_UPDATE_REQUEST) UE → Network uplinkDirectTransfer (NASCC ALERT) UE ← Network radioBearerReconfiguration UE → NetworkradioBearerReconfigurationComplete

←

→

UE ← Network securityModeCommand (PS domain) UE → NetworksecurityModeComplete UE ← Network downlinkDirectTransfer(GMM_ROUTING_AREA_UPDATE_ACCEPT) UE → Network uplinkDirectTransfer(ROUTING_AREA_UPDATE_COMPLETE) UE ← Network downlinkDirectTransfer (NASGMM GMM_INFORMATION) UE ← Network signallingConnectionRelease (PSdomain)

As seen in Table 4 above, the PS establishment does not start untilafter the UE sends a radioBearerSetupComplete message for the CS domainRAB to the network.

Another embodiment of the present disclosure is illustrated with respectto FIG. 3. In particular, in order not to increase PS bearerestablishment during a CSFB call when there is an ongoing active PS datasession on LTE network, a check may be made to determine whether toperform the CS voice radio bearer establishment in parallel with the PSbearer establishment, or whether the CS voice radio bearer establishmentmay occur first.

In particular, the method of FIG. 3 starts at block 300 and proceeds toblock 310 in which the UE determines whether a PS data session isongoing. In at least one embodiment, the UE also determines whetherthere are pending requests for a PS data session. In other embodiments,the determination at block 310 may simply be that the UE is in LTEactive. Other examples are possible.

From block 310 the process proceeds to block 320 in which the UE thenrequests a CSFB call from the network. As with the embodiment of FIG. 2,the request may be based on a page for a mobile terminated call or maybe based on a mobile originated call.

The process then proceeds to block 330, in which the UE receivesredirection information from the network in response to the request fora CSFB call.

The process then proceeds to block 335 in which a check is made whethera PS data session was ongoing or if there were pending requests for a PSdata session, as determined at block 310. If there was an ongoing PSdata session, or if there was a pending request for a PS data session,the process proceeds from block 335 to block 340 where circuit andpacket switched bearers are established in parallel, for example usingsignaling similar to that of Table 2 above. The process then proceeds toblock 350 and ends.

Conversely, from block 335, if there was no PS data session, or if therewas no pending request for a PS data session, the method proceeds toblock 360 to establish the CS voice radio bearers, and then to block 370to wait until it is determined that the circuit switched bearers areestablished. Once the circuit switched voice radio bearers areestablished, the process proceeds to block 380 in which the packetswitched bearers are established. From block 380 the process proceeds toblock 350 and ends. The process of block 360 and 380 may, for example,use the signaling of Table 4 above.

In at least some embodiments, the process of block 310, namelydetermining whether a PS data session is ongoing, could be performed atblock 330 instead.

In a further embodiment, rather than wait and check for CS voice radiobearers being (or in addition to this process), a delay may beintroduced after CS voice radio bearer establishment is initiated. Thefurther embodiment is illustrated with respect to FIG. 4.

The process of FIG. 4 starts at block 400 and proceeds to block 410 inwhich the UE makes a CSFB call request to the network. As indicatedabove for the embodiments of FIGS. 2 and 3, the request may be madebased on the receipt of a page for a mobile terminated call or based ona mobile originated call. From Tables 2 and 4, the mobile terminatedversion of the signaling may be a EPS MM Extended service request (MTCSFB) message.

From block 410 the process proceeds to block 420, in which the UEreceives redirection information from the network in response to therequest for a CSFB call. For example, from Table 4 the redirectioninformation may be in the form of message rrcConnection Release(redirectedCarrierinfo utra-FDD).

The process then proceeds to block 430, in which the UE acquires theUTRAN cell and establishes circuit switched voice radio bearers. Such CSvoice radio bearer establishment may be in the form of aradioBearerSetup (CS domain RAB) message.

The process then proceeds to block 440, where the UE waits apredetermined amount of time. The predetermined amount of time maybepreset, for example at the time the UE is manufactured or by a carrierwhen a device is deployed, or may be configured by the network, or byother means known in the art. The timer for the predetermined wait timeis reset if the CS voice radio bearer is established prior to itsexpiry.

After the predetermined amount of time has elapsed, the process proceedsto block 450, in which the procedure to transfer PS domain is initiated,for example, using the GMM_ROUTING_AREA_UPDATE_REQUEST. From block 450the process proceeds to block 460 and ends.

In some cases, the processes of FIG. 3 and FIG. 4 may be combined.Specifically, if a PS data session is ongoing, or if there is a pendingrequest for a PS data session at the time of the CSFB call, the UE mayskip waiting for a predetermined amount of time and proceed immediatelyto establishing packet switched bearers.

Furthermore, the predetermined amount of time could be set to a lengthwhich is longer than the expected time required for establishing circuitswitched voice radio bearers. Specifically, if for some reason the UE isunable to establish circuit switched voice radio bearers and the UE isprevented from establishing packet switched bearers until the circuitswitched voice radio bearers are established, the UE may becomeinoperable. Accordingly, in some embodiments, the method waits for thefirst one of the establishment of circuit switched voice radio bearersor the expiration of a predetermined time period to establish packetswitched bearers. In a further embodiment, the predetermined time periodis longer than an expected or an average time required to establishcircuit switched voice radio bearers.

In a further embodiment, the delaying procedure is performed on thenetwork side instead of on the UE side. Reference is now made to FIG. 5.

The process of FIG. 5 starts at block 500 and proceeds to block 510 inwhich the network element receives a CSFB call request from the UE.

The process then proceeds to block 520 in which the network elementresponds to the CSFB request with redirection information. From Table 4,the response may be a redirectedCarrierinfo utra-FDD message.

The process then proceeds to block 530, where the network elementreceives a Routing Area Update message from the UE. In at least someembodiments, the network element examines the follow-on bit in theRouting Area Update message to determine whether the UE is in an activePS data session or a PS data session is about to start.

If the network element determines that the UE is in an active datasession, as shown by block 535, the network element may proceed directlyfrom block 530 to block 550 and respond to the Routing Area Updatemessage, after which the method ends at block 560.

Otherwise, if the network element determines that the UE is not in anactive data session, or if the network element does not perform thecheck of block 535, the network element waits until the UE hasestablished its circuit switched voice radio bearers as shown at block540.

Once the network element determines that the UE has established circuitswitched voice radio bearers, the network element may respond to theRouting Area Update message at block 550, after which the process endsat block 560.

Therefore, the above embodiments provide for the delay, at least in somecases, of the establishment of PS radio bearers to reduce the call setuplatency of a CFSB call.

Practical results demonstrate a relative improvement for CSFB voice callsetup times, attributed to the above embodiments, shown in Table 5below. Table 5 further shows the increase in the time required totransfer the PS domain.

TABLE 5 Example Call Setup Time Comparison LTE Ext UMTS RRC Median timeof PS Service Connection data session Request to Request to transfer(ms) UMTS RRC UMTS Radio UMTS RRC Source to Connection Bearer SetupConnection Destination Request for CS domain Request → RAU Type NetworkType of Call (LTE→UMTS) messages Accept message Land to Network 1 LegacyCS N/A 1268.5 N/A Mobile Call CSFB 1000 1526 CSFB 929 1396 (modified)Network 2 Legacy CS N/A 1545 N/A CSFB 988.5 2418 2346 CSFB 1071.5 17203694 (modified) Network 3 Legacy CS N/A 1691.5 N/A CSFB 1080 2661 1986CSFB 860 1746 3815.5 (modified) Mobile to Network 3 Legacy CS N/A 3749N/A Mobile Call CSFB 920 5850 2296 CSFB 954 4296 6357.5 (modified)

In Table 5, a “legacy CS” is a call originated on a UTRAN cell. The“CSFB” is the time using existing CSFB procedures and “CSFB (modified)”uses the embodiment of FIG. 2 above. From the table the modified CSFBprocedure of FIG. 2 provides a reduction in the call setup time.

Further results are illustrated in FIG. 6, which compares the timerequired to establish a legacy CS voice call, a prior art CSFB voicecall, and a CSFB voice call in accordance with the present disclosure.The label T1 stands for the time interval between the CSFB request fromthe UE when the UE is on an LTE network and the UMTS RRC ConnectionRequest after the UE has acquired a UMTS network. The label T2 standsfor the time interval between the UMTS RRC Connection Request and theUMTS CS voice radio bearer setup. The label Total stands for the totalof T1 and T2.

The above embodiments may be implemented by any UE. One exemplary deviceis described below with regard to FIG. 7.

UE 700 is typically a two-way wireless communication device having voiceand/or data communication capabilities. UE 700 generally has thecapability to communicate with other computer systems on the Internet.Depending on the exact functionality provided, the UE may be referred toas a data messaging device, a two-way pager, a wireless e-mail device, acellular telephone with data messaging capabilities, a wireless Internetappliance, a wireless device, a mobile device, or a data communicationdevice, as examples.

Where UE 700 is enabled for two-way communication, it may incorporate acommunication subsystem 711, including both a receiver 712 and atransmitter 714, as well as associated components such as one or moreantenna elements 716 and 718, local oscillators (LOs) 713, and aprocessing module such as a digital signal processor (DSP) 720. As willbe apparent to those skilled in the field of communications, theparticular design of the communication subsystem 711 will be dependentupon the communication network in which the device is intended tooperate. The radio frequency front end of communication subsystem 711can be for any of the embodiments described above.

Network access requirements will also vary depending upon the type ofnetwork 719. In some networks network access is associated with asubscriber or user of UE 700. A UE may require a removable user identitymodule (RUIM) or a subscriber identity module (SIM) or a universalsubscriber identity module (USIM) in order to operate on a network. TheSIM/USIM/RUIM interface 744 is normally similar to a card-slot intowhich a SIM/RUIM card can be inserted and ejected. The SIM/USIM/RUIMcard can have memory and hold many key configurations 751, and otherinformation 753 such as identification, and subscriber relatedinformation.

When required network registration or activation procedures have beencompleted, UE 700 may send and receive communication signals over thenetwork 719. As illustrated in FIG. 7, network 719 can consist ofmultiple base stations communicating with the UE. Further, as describedabove, a multi-mode UE may communicate with base stations of differentnetworks 719, for example a WCDMA and an LTE network.

Signals received by antenna 716 through communication network 719 areinput to receiver 712, which may perform such common receiver functionsas signal amplification, frequency down conversion, filtering, channelselection and the like. A/D conversion of a received signal allows morecomplex communication functions such as demodulation and decoding to beperformed in the DSP 720. In a similar manner, signals to be transmittedare processed, including modulation and encoding for example, by DSP 720and input to transmitter 714 for digital to analog conversion, frequencyup conversion, filtering, amplification and transmission over thecommunication network 719 via antenna 718. DSP 720 not only processescommunication signals, but also provides for receiver and transmittercontrol. For example, the gains applied to communication signals inreceiver 712 and transmitter 714 may be adaptively controlled throughautomatic gain control algorithms implemented in DSP 720.

UE 700 generally includes a processor 738 which controls the overalloperation of the device. Communication functions, including data andvoice communications, are performed through communication subsystem 711.Processor 738 also interacts with further device subsystems such as thedisplay 722, flash memory 724, random access memory (RAM) 726, auxiliaryinput/output (I/O) subsystems 728, serial port 730, one or morekeyboards or keypads 732, speaker 734, microphone 736, othercommunication subsystem 740 such as a short-range communicationssubsystem and any other device subsystems generally designated as 742.Serial port 730 could include a USB port or other port known to those inthe art.

Some of the subsystems shown in FIG. 7 perform communication-relatedfunctions, whereas other subsystems may provide “resident” or on-devicefunctions. Notably, some subsystems, such as keyboard 732 and display722, for example, may be used for both communication-related functions,such as entering a text message for transmission over a communicationnetwork, and device-resident functions such as a calculator or tasklist.

Operating system software used by the processor 738 may be stored in apersistent store such as flash memory 724, which may instead be aread-only memory (ROM) or similar storage element (not shown). Thoseskilled in the art will appreciate that the operating system, specificdevice applications, or parts thereof, may be temporarily loaded into avolatile memory such as RAM 726. Received communication signals may alsobe stored in RAM 726.

As shown, flash memory 724 can be segregated into different areas forboth computer programs 758 and program data storage 750, 752, 754 and756. These different storage types indicate that each program canallocate a portion of flash memory 724 for their own data storagerequirements. Processor 738, in addition to its operating systemfunctions, may enable execution of software applications on the UE. Apredetermined set of applications that control basic operations,including at least data and voice communication applications forexample, will normally be installed on UE 700 during manufacturing.Other applications could be installed subsequently or dynamically.

Applications and software may be stored on any computer readable storagemedium. The computer readable storage medium may be a tangible or intransitory/non-transitory medium such as optical (e.g., CD, DVD, etc.),magnetic (e.g., tape) or other memory known in the art.

One software application may be a personal information manager (PIM)application having the ability to organize and manage data itemsrelating to the user of the UE such as, but not limited to, e-mail,calendar events, voice mails, appointments, and task items. Naturally,one or more memory stores would be available on the UE to facilitatestorage of PIM data items. Such PIM application may have the ability tosend and receive data items, via the wireless network 719. Furtherapplications may also be loaded onto the UE 700 through the network 719,an auxiliary I/O subsystem 728, serial port 730, short-rangecommunications subsystem 740 or any other suitable subsystem 742, andinstalled by a user in the RAM 726 or a non-volatile store (not shown)for execution by the processor 738. Such flexibility in applicationinstallation increases the functionality of the device and may provideenhanced on-device functions, communication-related functions, or both.For example, secure communication applications may enable electroniccommerce functions and other such financial transactions to be performedusing the UE 700.

In a data communication mode, a received signal such as a text messageor web page download will be processed by the communication subsystem711 and input to the processor 738, which may further process thereceived signal for output to the display 722, or alternatively to anauxiliary I/O device 728.

A user of UE 700 may also compose data items such as email messages forexample, using the keyboard 732, which may be a complete alphanumerickeyboard or telephone-type keypad, or a virtual keyboard, among others,in conjunction with the display 722 and possibly an auxiliary I/O device728. Such composed items may then be transmitted over a communicationnetwork through the communication subsystem 711.

For voice communications, overall operation of UE 700 is similar, exceptthat received signals would typically be output to a speaker 734 andsignals for transmission would be generated by a microphone 736.Alternative voice or audio I/O subsystems, such as a voice messagerecording subsystem, may also be implemented on UE 700. Although voiceor audio signal output is generally accomplished primarily through thespeaker 734, display 722 may also be used to provide an indication ofthe identity of a calling party, the duration of a voice call, or othervoice call related information for example.

Serial port 730 in FIG. 7 may be implemented in a personal digitalassistant (PDA)-type UE for which synchronization with a user's desktopcomputer (not shown) may be desirable, but is an optional devicecomponent. Such a port 730 would enable a user to set preferencesthrough an external device or software application and would extend thecapabilities of UE 700 by providing for information or softwaredownloads to UE 700 other than through a wireless communication network.The alternate download path may for example be used to load anencryption key onto the device through a direct and thus reliable andtrusted connection to thereby enable secure device communication. Aswill be appreciated by those skilled in the art, serial port 730 canfurther be used to connect the UE to a computer to act as a modem or forcharging purposes.

Other communications subsystems 740, such as a short-rangecommunications subsystem, is a further optional component which mayprovide for communication between UE 700 and different systems ordevices, which need not necessarily be similar devices. For example, thesubsystem 740 may include an infrared device and associated circuits andcomponents or a Bluetooth™ communication module to provide forcommunication with similarly enabled systems and devices. Subsystem 740may further include non-cellular communications such as WiFi or WiMAX,or near field communications (NFC).

Further, the network elements in the embodiments above can be anynetwork element, or part of any network element, including variousnetwork servers. Reference is now made to FIG. 8, which shows ageneralized network element.

In FIG. 8, network element 810 includes a processor 820 and acommunications subsystem 830, where the processor 820 and communicationssubsystem 830 cooperate to perform the methods of the embodimentsdescribed above.

Processor 820 is configured to execute programmable logic, which may bestored, along with data, on network element 810, and shown in theexample of FIG. 8 as memory 840. Memory 840 can be any tangible storagemedium.

Alternatively, or in addition to memory 840, network element 810 mayaccess data or programmable logic from an external storage medium, forexample through communications subsystem 830.

Communications subsystem 830 allows network element 810 to communicatewith other network elements or to UEs.

Communications between the various elements of network element 810 maybe through an internal bus 850 in one embodiment. However, other formsof communication are possible.

The embodiments described herein are examples of structures, systems ormethods having elements corresponding to elements of the techniques ofthis application. This written description may enable those skilled inthe art to make and use embodiments having alternative elements thatlikewise correspond to the elements of the techniques of thisapplication. The intended scope of the techniques of this applicationthus includes other structures, systems or methods that do not differfrom the techniques of this application as described herein, and furtherincludes other structures, systems or methods with insubstantialdifferences from the techniques of this application as described herein.

1. A method, at a user equipment (‘UE’), comprising: sending a requestto a first network for Circuit Switched Fallback (‘CSFB’) whileconnected to the first network; receiving, responsive to the request,redirection information to a second network; acquiring the secondnetwork based on the redirection information; initiating Packet Switched(‘PS’) signaling with the second network after radio bearers for acircuit switched (CS) connection are established with the secondnetwork.
 2. The method of claim 1, wherein the initiating begins afterexpiration of a timer.
 3. The method of claim 1, wherein the initiatingbegins after a first of CS voice radio bearers being established andexpiration of a timer.
 4. The method of claim 1, wherein the UE waitsfor a predetermined event to occur before said initiating if the UE doesnot have a pending data session request on the second network.
 5. Themethod of claim 1, wherein the UE waits for a predetermined event tooccur before said initiating if the UE does not have a pending datatransmission on the second network.
 6. The method of claim 1, wherein CSvoice radio bearers are determined to be established when the UE sends aradioBearerSetupComplete message for the CS domain.
 7. The method ofclaim 2, wherein an initial value of the timer is configured based on amessage from one of the first network and the second network.
 8. Themethod of claim 3, wherein an initial value of the timer is greater thanan expected time required for the establishment of CS voice radiobearers.
 9. The method of claim 2, wherein the timer is started uponinitiating establishment of the CS voice radio bearers.
 10. A userequipment comprising: a processor; a communications subsystem; whereinthe user equipment is configured: send a request to a first network forCircuit Switched Fallback (‘CSFB’) while connected to the first network;receive, responsive to the request, redirection information to a secondnetwork; acquire the second network based on the redirectioninformation; initiate Packet Switched (‘PS’) signaling with the secondnetwork after radio bearers for a circuit switched (CS) connection areestablished with the second network.
 11. The user equipment of claim 10,wherein the initiating begins after expiration of a timer.
 12. The userequipment of claim 10, wherein the initiating begins after apredetermined event being a first one of CS voice radio bearers beingestablished and expiration of a timer.
 13. The user equipment of claim10, wherein the UE waits for a predetermined event to occur before theinitiating if the UE does not have a pending data session request on thesecond network
 14. The user equipment of claim 10, wherein the UE waitsfor a predetermined event to occur before the initiating if the UE doesnot have a pending data transmission on the second network.
 15. The userequipment of claim 10, wherein CS voice radio bearers are determined tobe established when the UE sends a radioBearerSetupComplete message forthe CS domain.
 16. The user equipment of claim 11, wherein an initialvalue of the timer is configured based on a message from one of thefirst network and the second network.
 17. The user equipment of claim12, wherein an initial value of the timer is greater than an expectedtime required for the establishment of CS voice radio bearers.
 18. Theuser equipment of claim 11, wherein the timer is started upon initiatingthe establishment of CS voice radio bearers.
 19. A method, at a networkelement, comprising: receiving a request for Circuit Switched Fallback(‘CSFB’) from a user equipment (‘UE’), the UE having a connection to afirst network; sending, to the UE, redirection information for transferof the connection to a second network; receiving a Routing Area Update(‘RAU’) message from the UE to initiate the transfer of the connectionto the second network; initiating Packet Switched (‘PS’) signaling withthe second network after radio bearers for a circuit switched (CS)connection are established with the second network
 20. A networkelement, comprising: a processor; a communications subsystem; whereinthe network element is configured to: send a request to a first networkfor Circuit Switched Fallback (‘CSFB’) while connected to the firstnetwork; receive, responsive to the request, redirection information toa second network; acquire the second network based on the redirectioninformation; initiate Packet Switched (‘PS’) signaling with the secondnetwork after radio bearers for a circuit switched (CS) connection areestablished with the second network.